If I were to purchase a brand new new 3ds xl red or black from amazon, how likely is it that the firmware it comes with be less than 11?
I got a brand new N3DS LL pearl white from play-asia and it was on 10.7. I think you will be <11.0, but can't guarantee it.
Depending on the vendor you could always e-mail and ask them for the version number, then lookup here: https://www.reddit.com/r/3dshacks/comments/4pmpgb/new_3ds_serial_firmware_list/